Semiconductor device and method for fabricating the same

ABSTRACT

A method for fabricating a semiconductor device includes the steps of: forming a fin-shaped structure on a substrate, forming a gate material layer on the fin-shaped structure, performing an etching process to pattern the gate material layer for forming a gate structure and a silicon residue, performing an ashing process on the silicon residue, and then performing a cleaning process to transform the silicon residue into a polymer stop layer on a top surface and sidewalls of the fin-shaped structure.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for fabricating semiconductor device, and more particularly to a method of forming polymer stop layer adjacent to a gate structure.

2. Description of the Prior Art

With the trend in the industry being towards scaling down the size of the metal oxide semiconductor transistors (MOS), three-dimensional or non-planar transistor technology, such as fin field effect transistor technology (FinFET) has been developed to replace planar MOS transistors. Since the three-dimensional structure of a FinFET increases the overlapping area between the gate and the fin-shaped structure of the silicon substrate, the channel region can therefore be more effectively controlled. This way, the drain-induced barrier lowering (DIBL) effect and the short channel effect are reduced. The channel region is also longer for an equivalent gate length, thus the current between the source and the drain is increased. In addition, the threshold voltage of the FinFET can be controlled by adjusting the work function of the gate.

As the semiconductor industry enters 10 nm node generation, the resistance value of metal gates plays an important role in the performance of the FinFET device. Since metal gate transistor architecture today is still insufficient in achieving desirable performance, how to improve the design of current transistor structure has become an important task in this field.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method for fabricating a semiconductor device includes the steps of: forming a fin-shaped structure on a substrate, forming a gate material layer on the fin-shaped structure, performing an etching process to pattern the gate material layer for forming a gate structure and a silicon residue, performing an ashing process on the silicon residue, and then performing a cleaning process to transform the silicon residue into a polymer stop layer on a top surface and sidewalls of the fin-shaped structure.

According to another aspect of the present invention, a semiconductor device includes a fin-shaped structure on a substrate, a gate structure on the fin-shaped structure, and a polymer stop layer on a top surface and sidewalls of the fin-shaped structure adjacent to the gate structure.

According to yet another aspect of the present invention, a semiconductor device includes a fin-shaped structure extending along a first direction on a substrate according to a top view, a first gate structure extending along a second direction adjacent to one side of the fin-shaped structure and a second gate structure extending along the second direction adjacent to another side of the fin-shaped structure. Preferably, a first corner of the first gate structure adjacent to the fin-shaped structure includes a first polymer stop layer;

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 illustrate a method for fabricating semiconductor device according to an embodiment of the present invention.

FIG. 7 illustrates a structural top view of a semiconductor device according to an embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1-6 , FIGS. 1-6 illustrate a method for fabricating semiconductor device according to an embodiment of the present invention. As shown in FIG. 1 , a substrate 12 is first provided, in which the substrate could be a silicon substrate or silicon-on-insulator (SOI) substrate. A transistor region, such as a PMOS region or a NMOS region could be defined on the substrate 12. At least a fin-shaped structure 14 and an insulating layer (not shown) are formed on the substrate 12, in which the bottom portion of the fin-shaped structure 14 is surrounded by the insulating layer preferably made of dielectric material such as silicon oxide to form a shallow trench isolation (STI) 16. It should be noted that even though this embodiment pertains to the fabrication of a non-planar FET device such as FinFET device, it would also be desirable to apply the following process to a planar FET device, which is also within the scope of the present invention.

According to an embodiment of the present invention, the fin-shaped structure 14 could be obtained by a sidewall image transfer (SIT) process. For instance, a layout pattern is first input into a computer system and is modified through suitable calculation. The modified layout is then defined in a mask and further transferred to a layer of sacrificial layer on a substrate through a photolithographic and an etching process. In this way, several sacrificial layers distributed with a same spacing and of a same width are formed on a substrate. Each of the sacrificial layers may be stripe-shaped. Subsequently, a deposition process and an etching process are carried out such that spacers are formed on the sidewalls of the patterned sacrificial layers. In a next step, sacrificial layers can be removed completely by performing an etching process. Through the etching process, the pattern defined by the spacers can be transferred into the substrate underneath, and through additional fin cut processes, desirable pattern structures, such as stripe patterned fin-shaped structures could be obtained.

Alternatively, the fin-shaped structure 14 could also be obtained by first forming a patterned mask (not shown) on the substrate, 12, and through an etching process, the pattern of the patterned mask is transferred to the substrate 12 to form the fin-shaped structure. Moreover, the formation of the fin-shaped structure could also be accomplished by first forming a patterned hard mask (not shown) on the substrate 12, and a semiconductor layer composed of silicon germanium is grown from the substrate 12 through exposed patterned hard mask via selective epitaxial growth process to form the corresponding fin-shaped structure. These approaches for forming fin-shaped structure are all within the scope of the present invention.

Next, at least a gate structure 18 or dummy gate is formed on the substrate 12. In this embodiment, the formation of the gate structure 18 could be accomplished by a gate first process, a high-k first approach from gate last process, or a high-k last approach from gate last process. Since this embodiment pertains to a high-k last approach, a gate dielectric layer or interfacial layer 20, a gate material layer 22 preferably made of polysilicon, a hard mask 24, and another hard mask 26 could be formed sequentially on the substrate 12, and a pattern transfer process is then conducted by using a patterned resist (not shown) as mask to remove part of the hard masks 26, 24, part of the gate material layer 22 and part of the gate dielectric layer through single or multiple etching processes. After stripping the patterned resist, a gate structure 18 composed of an un-patterned interfacial layer 20, a patterned gate material layer 22 or gate electrode layer, and patterned hard masks 24, 26 are formed on the substrate 12. In this embodiment, the interfacial layer 20 is preferably made of silicon oxide, the hard mask 24 is made of silicon nitride, and the hard mask 26 is made of silicon oxide.

It should be noted that the pattern transfer process conducted to form the gate structure 18 is preferably accomplished by first conducting an etching process to pattern the hard masks 24, 26, the gate material layer 22, and even the interfacial layer 20 for forming the gate structure 18 and at the same time forming silicon residue (not shown) on the top surface and sidewalls of the fin-shaped structure 14 adjacent to two sides of the gate structure 18, in which the etching process could include gases such as but not limited to for example carbon tetrafluoride (CF₄), carbon tetrachloride (CCl₄), nitrogen gas (N₂), difluoromethane (CH₂F₂), dibromomethane (CH₂Br₂), and/or dichloromethane (CH₂Cl₂). Preferably, the etching process forms fluorine gas (F₂) and a silicon residue made of SiCNF_(x) or silicon residue containing elements such as carbon, nitrogen, and fluorine.

Next, an ashing process is conducted to transform the silicon residue originally made of SiCNF_(x) through oxygen plasma into SiCONF. Next, a cleaning process is conducted by using hydrogen peroxide (H₂O₂) to transform the silicon residue including SiCONF into a polymer stop layer 28, in which the polymer stop layer 28 includes elements including carbon, oxygen, nitrogen, hydrogen, and fluorine or more specifically a compound made of SiCONHF. In other words, the patterning process conducted to form the gate structure 18 preferably includes the aforementioned etching process used to form silicon residue, the ashing process, and the cleaning process and during the formation of the gate structure 18, the polymer stop layer 28 is formed at the same time on the top surface and sidewalls of the fin-shaped structure 14 adjacent to two sides of the gate structure 18.

Referring to FIGS. 1-2 , FIG. 2 illustrates a 3-dimensional view for fabricating a semiconductor device according to an embodiment of the present invention and FIG. 1 illustrates a structural view of the semiconductor device viewing along the direction of the arrow as shown in FIG. 2 . It should be noted that the portion labeled with fin-shaped structure 14 between two polymer stop layers 28 shown in FIG. 1 is in fact the overlapped portion when the gate structure 18 stands astride the fin-shaped structure 14. However for simplicity purpose this portion is only labeled with fin-shaped structure 14. As shown in FIGS. 1-2 , the polymer stop layers 28 formed through the aforementioned etching process, ashing process, and cleaning process is preferably formed on the top surface and sidewalls of the fin-shaped structure 14 adjacent to two sides of the gate structure 18. Viewing from a more detailed perspective, the polymer stop layers 28 are disposed from the top surface of the fin-shaped structure 14 adjacent to two sides of the gate structure 18, extending downward along sidewalls of the fin-shaped structure 14 adjacent to two sides of the fin-shaped structure 14, and stop until reaching a boundary between the bottom surface of the fin-shaped structure 14 and the STI 16. In other words, each of the polymer stop layers 28 shown in FIG. 2 preferably includes a substantially reverse U-shape sitting on the fin-shaped structure 14 adjacent to two sides of the gate structure 18, in which the polymer stop layers 28 are only disposed on the surface of the fin-shaped structure 14 adjacent to two sides of the gate structure 18 but not directly under the gate structure 18 or between the fin-shaped structure 14 and the gate structure 18.

Next, as shown in FIG. 3 , a re-oxidation process is conducted to form a first seal layer 30 on sidewalls of the gate structure 18. Specifically, the re-oxidation process conducted at this stage is accomplished by using furnace or dry etching approach to inject oxygen gas under approximately 750° C. to form a first seal layer 30 made of silicon oxide on top surface and sidewalls of the gate structure 18, including sidewalls of the gate material layer 22, sidewalls of the hard mask 24, and sidewalls and top surface of the hard mask 26. It should be noted that the oxygen gas applied throughout the re-oxidation process preferably penetrates or diffuses through the polymer stop layers 28 to oxidize all of the sidewalls of the gate structure 18. In other words, the first seal layer 30 is preferably formed on the inner sidewalls of the polymer stop layers 28 or if viewed from another perspective, the inner sidewalls of the first seal layer 30 preferably contact the gate structure 18 directly while the outer sidewalls of the first seal layer 30 contacts the inner sidewalls of the polymer stop layers 28 directly.

Next, as shown in FIG. 4 , a second seal layer 32 is formed on sidewalls of the first seal layer 30 and the polymer stop layers 28, and an etching back process is conducted to remove part of the second seal layer 32 and part of the first seal layer 30 to form a first spacer 34 and a second spacer 36 on sidewalls of the gate structure 18. In this embodiment, the first spacer 34 and the second spacer 36 are preferably made of different materials, in which the first spacer 34 is preferably made of silicon oxide while the second spacer 36 is made of silicon oxycarbonitride (SiOCN). Nevertheless, according to other embodiments of the present invention, the second spacer 36 could also be selected from the group consisting of silicon oxide, silicon nitride (SiN), silicon oxynitride (SiON), and silicon carbon nitride (SiCN).

Next, a dry etching and/or wet etching process is conducted by using the gate structure 18 and second spacer 36 as mask to remove part of the substrate 12 along the second spacer 36 to form recesses (not shown) adjacent to two sides of the gate structure 18. Next, a selective epitaxial growth (SEG) process is conducted to form an epitaxial layer 38 in each of the recesses. In this embodiment, a top surface of the epitaxial layer 38 is preferably even with a top surface of the substrate 12, in which the epitaxial layer 38 also shares substantially same cross-section shape with the recess. For instance, the cross-section of the epitaxial layer 38 could also a circle, a hexagon, or an octagon depending on the demand of the product. In this embodiment, the epitaxial layer 38 could also be formed to include different material depending on the type of transistor being fabricated. For instance, if the MOS transistor being fabricated were to be a PMOS transistor, the epitaxial layer 38 could be made of material including but not limited to for example SiGe, SiGeB, or SiGeSn. If the MOS transistor being fabricated were to be a NMOS transistor, the epitaxial layer 38 could be made of material including but not limited to for example SiC, SiCP, or SiP. Moreover, the SEG process could also be adjusted to form a single-layered epitaxial structure or multi-layered epitaxial structure, in which heteroatom such as germanium atom or carbon atom of the structure could be formed to have gradient while the surface of the epitaxial layer 38 is preferred to have less or no germanium atom at all to facilitate the formation of silicide afterwards. It should be noted that even though the top surfaces of the fin-shaped structure 14 and epitaxial layer 38 are coplanar in this embodiment, it would also be desirable extend the epitaxial layer 38 upward so that the top surface of the epitaxial layer 38 is higher than the top surface of the fin-shaped structure 14 according to another embodiment of the present invention.

Next, an ion implantation process is conducted to form a source/drain region 40 in part or the entire epitaxial layer 38. According to an embodiment of the present invention, the source/drain region 40 could also be formed insituly during the SEG process. For instance, the source/drain region 40 could be formed by implanting p-type dopants during formation of a SiGe epitaxial layer, a SiGeB epitaxial layer, or a SiGeSn epitaxial layer for PMOS transistor, or could be formed by implanting n-type dopants during formation of a SiC epitaxial layer, SiCP epitaxial layer, or SiP epitaxial layer for NMOS transistor. By doing so, it would be desirable to eliminate the need for conducting an extra ion implantation process for forming the source/drain region. Moreover, the dopants within the source/drain region 40 could also be formed with a gradient, which is also within the scope of the present invention.

Next, as shown in FIG. 5 , a contact etch stop layer (CESL) 42 is formed on the substrate 12 surface and the gate structure 18, and an interlayer dielectric (ILD) layer 44 is formed on the CESL 42 afterwards. Next, a planarizing process such as a chemical mechanical polishing (CMP) process is conducted to remove part of the ILD layer 44 and part of the CESL 42 to expose the hard mask 26 so that the top surfaces of the hard mask 26 and ILD layer 44 are coplanar. In this embodiment, the CESL 42 could include silicon nitride while the ILD layer 44 could include silicon oxide, but not limited thereto.

Next, as shown in FIG. 6 , a replacement metal gate (RMG) process is conducted to transform the gate structure 18 into a metal gate 46. For instance, the RMG process could be accomplished by first performing a selective dry etching or wet etching process using etchants including but not limited to for example ammonium hydroxide (NH₄OH) or tetramethylammonium hydroxide (TMAH) to remove the hard masks 24, 26 and gate material layer 22 for forming a recess (not shown) in the ILD layer 44. Next, a high-k dielectric layer 48, a work function metal layer 50, and a low resistance metal layer 52 are formed in the recess, and a planarizing process such as CMP is conducted to remove part of low resistance metal layer 52, part of work function metal layer 50, and part of high-k dielectric layer 48 to form metal gate 46. In this embodiment, the gate structure or metal gate 46 fabricated through high-k last process of a gate last process preferably includes an interfacial layer 20 or gate dielectric layer, a U-shaped high-k dielectric layer 48, a U-shaped work function metal layer 50, and a low resistance metal layer 52. Next, part of the low resistance metal layer 52, part of the work function metal layer 50, and part of the high-k dielectric layer 48 are removed to form a recess, a hard mask 54 is formed in the recess, and a planarizing process such as CMP is conducted to remove part of the hard mask 54 so that the top surfaces of the hard mask 54 and ILD layer 44 are coplanar.

In this embodiment, the high-k dielectric layer 48 is preferably selected from dielectric materials having dielectric constant (k value) larger than 4. For instance, the high-k dielectric layer 48 may be selected from hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO₄), hafnium silicon oxynitride (HfSiON), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃), tantalum oxide (Ta₂O₅), yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), strontium titanate oxide (SrTiO₃), zirconium silicon oxide (ZrSiO₄), hafnium zirconium oxide (HfZrO₄), strontium bismuth tantalate (SrBi₂Ta₂O₉, SBT), lead zirconate titanate (PbZr_(x)Ti_(1-x)O₃, PZT), barium strontium titanate (Ba_(x)Sr_(1-x)TiO₃, BST) or a combination thereof.

In this embodiment, the work function metal layer 50 is formed for tuning the work function of the metal gate in accordance with the conductivity of the device. For an NMOS transistor, the work function metal layer 50 having a work function ranging between 3.9 eV and 4.3 eV may include titanium aluminide (TiAl), zirconium aluminide (ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), or titanium aluminum carbide (TiAlC), but it is not limited thereto. For a PMOS transistor, the work function metal layer 50 having a work function ranging between 4.8 eV and 5.2 eV may include titanium nitride (TiN), tantalum nitride (TaN), tantalum carbide (TaC), but it is not limited thereto. An optional barrier layer (not shown) could be formed between the work function metal layer 50 and the low resistance metal layer 52, in which the material of the barrier layer may include titanium (Ti), titanium nitride (TiN), tantalum (Ta) or tantalum nitride (TaN). Furthermore, the material of the low-resistance metal layer 52 may include copper (Cu), aluminum (Al), titanium aluminum (TiAl), cobalt tungsten phosphide (CoWP) or any combination thereof. Next, a contact plug formation could be conducted by forming contact plugs 56 in the ILD layer 44 and CESL 42 to electrically connect the source/drain region 40 adjacent to two sides of the second spacer 36. This completes the fabrication of a semiconductor device according to an embodiment of the present invention.

Referring again to FIGS. 2 and 6 , FIGS. 2 and 6 illustrate structural views of a semiconductor device according to an embodiment of the present invention. As shown in FIGS. 2 and 6 , the semiconductor device preferably includes a gate structure 18 disposed on the substrate 12, a first spacer 34 disposed on sidewalls of the gate structure 18, a second spacer 36 disposed on sidewalls of the first spacer 34, a polymer stop layer 28 disposed between the first spacer 34 and the second spacer 36, and a source/drain region 40 dispose adjacent to two sides of the gate structure 18. Preferably, the polymer stop layer 28 is disposed on the top surface and sidewalls of the fin-shaped structure 14 adjacent to two sides of the gate structure 18.

As shown in the 3D structural view in FIG. 2 , even after the first spacer 34, the second spacer 36, and the source/drain regions 40 are formed the polymer stop layers 28 are still disposed from the top surface of the fin-shaped structure 14 adjacent to two sides of the gate structure 18 and extending along the two sidewalls of the fin-shaped structure 14 adjacent to two sides of the gate structure 18 downward till reaching the boundary between bottom surface of the fin-shaped structure 14 and the STI 16. As shown in the cross-sectional view in FIG. 6 , the polymer stop layers 28 are preferably disposed between the first spacer 34 and the second spacer 36, and even though each of the polymer stop layers 28 includes a triangular tip portion in this embodiment, according to other embodiment of the present invention, the tip shape of the polymer stop layers 28 could also vary depending on the gas used during the aforementioned etching process. For instance, the tip portion of each of the polymer stop layers 28 could include circular, rectangular square, or other irregular shapes. Preferably, the overall height of each of the polymer stop layers 28 is less than 1/10 or even 1/20 of the entire height of the gate structure 18.

Regarding the material of the aforementioned elements, the first spacer 34 and the polymer stop layers 28 are preferably made of different materials and the second spacer 36 and the polymer stop layers 28 are made of different materials, in which the first spacer 34 preferably includes silicon oxide, the second spacer 36 includes SiOCN, and the polymer stop layers 28 includes elements including carbon, oxygen, nitrogen, hydrogen, and fluorine or more specifically a compound made of SiCONHF.

Referring to FIG. 7 , FIG. 7 illustrates a structural top view of a semiconductor device according to an embodiment of the present invention. As shown in FIG. 7 , it would be desirable to first carry out the aforementioned RMG process for transforming the gate structure 18 into a metal gate 46 and as the planarizing process is conducted, all of the gate structure 18 or metal gate 46 directly on top of the fin-shaped structure 14 could be removed through a CMP process so that the gate structure 18 standing astride the fin-shaped structure 14 would be divided into portions, such as the gate structure 62 and gate structure 64 adjacent to two sides of the fin-shaped structure 14 as shown in FIG. 7 . In other words, the gate structure 18 standing astride the fin-shaped structure 14 is divided into two portions adjacent to two sides of the fin-shaped structure 14 through planarizing process while no gate structure 18 is remained directly on top of the fin-shaped structure 14. The polymer stop layers 28 originally disposed adjacent to two sides of the gate structure 18 are then divided into four portions or four polymer stop layers 66, 68, 70, 72 under a top view perspective after the planarizing process and the four portions 66, 68, 70, 72 are now disposed on four corners between the fin-shaped structure 14 and the gate structure 62, 64.

Viewing from an overall perspective, the fin-shaped structure 14 is disposed extending along a first direction such as Y-direction on the substrate 12, no gate structure 18 is disposed directly on top of the fin-shaped structure 14, the gate structure 62 formed from the gate structure 18 is extending along a second direction such as X-direction adjacent to one side of the fin-shaped structure 14, a gate structure 64 also formed from the gate structure 18 is extending along the second direction such as X-direction adjacent to another side of the fin-shaped structure 14, and interfacial layers 20 disposed between the fin-shaped structure 14 and the gate structures 62, 64.

Preferably, a first corner of the gate structure 62 immediately adjacent to the fin-shaped structure 14 includes a polymer stop layer 66, a second corner of the gate structure 62 immediately adjacent to the fin-shaped structure 14 includes a polymer stop layer 68, a third corner of the gate structure 64 immediately adjacent to the fin-shaped structure 14 includes a polymer stop layer 70, a fourth corner of the gate structure 64 immediately adjacent to the fin-shaped structure 14 includes a polymer stop layer 72, and the second spacer 36 is disposed around the fin-shaped structure 14 and the gate structures 62, 64.

In this embodiment, each of the polymer stop layers 66, 68, 70, 72 disposed on corners of the gate structures 62, 64 preferably contact the fin-shaped structures 14, the interfacial layer 20, and the gate structures 62, 64 directly under a top view perspective. It should be noted that even though each of the polymer stop layers 66, 68, 70, 72 includes a triangle shape under the top view perspective in this embodiment, according to other embodiment of the present invention, the shape of the polymer stop layers 66, 68, 70, 72 could also vary depending on the gas used during the aforementioned etching process. For instance, the top view shape of the polymer stop layers 66, 68, 70, 72 could include circles, rectangles, square, or other irregular shapes. Preferably, the polymer stop layers 66, 68, 70, 72 include same material or elements including carbon, oxygen, nitrogen, hydrogen, and fluorine or more specifically a compound made of SiCONHF.

In current FinFET process, polymer blocks or fin corner oxide (FCO) residues are often formed on corners between gate structure and the substrate during formation of the gate structure, and the polymer blocks are then sealed between gate electrode and the spacer formed afterwards. Since the polymer blocks in current process are typically sealed immediately adjacent to or directly contacting the gate material layer or gate electrode made of polysilicon, the polymer blocks are often removed along with the polysilicon gate material layer during the RMG process and results in severe leakage. To resolve this issue, the present invention first forms polymer stop layers on top surface and sidewalls of the fin-shaped structure adjacent to two sides of the gate structure, and then conducts an oxidation process to form a first seal layer 30 or first spacer 34 between sidewalls of the gate structure and inner sidewall of the polymer stop layers. By using the first spacer 34 as a barrier structure between the polymer stop layers and the gate structure, it would be desirable to prevent leakage when gate material layer made of polysilicon is removed during RMG process.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A method for fabricating a semiconductor device, comprising: forming a fin-shaped structure on a substrate; forming a gate material layer on the fin-shaped structure; and patterning the gate material layer to form a gate structure and at the same time forming a polymer stop layer on a top surface and sidewalls of the fin-shaped structure, wherein a top surface of the polymer stop layer is lower than a top surface of the gate structure.
 2. The method of claim 1, further comprising: performing an etching process to pattern the gate material layer for forming a silicon residue; performing an ashing process on the silicon residue; and performing a cleaning process to transform the silicon residue into the polymer stop layer.
 3. The method of claim 2, wherein the silicon residue comprises carbon, nitrogen, and fluorine.
 4. The method of claim 1, wherein the polymer stop layer comprises carbon, oxygen, nitrogen, hydrogen, and fluorine.
 5. The method of claim 1, further comprising: forming a first hard mask on the gate material layer; forming a second hard mask on the first hard mask; and patterning the second hard mask, the first hard mask, and the gate material layer to form the gate structure and the polymer stop layer.
 6. The method of claim 1, further comprising performing an oxidation process to oxidize sidewalls of the gate structure to form a first seal layer.
 7. The method of claim 6, further comprising forming the first seal layer on sidewalls of the gate structure and an inner sidewall of the polymer stop layer.
 8. The method of claim 6, further comprising: forming a second seal layer on sidewalls of the first seal layer and the polymer stop layer; removing the second seal layer and the first seal layer to form a first spacer and a second spacer; forming a source/drain region adjacent to two sides of the second spacer; forming an interlayer-dielectric (ILD) layer on the gate structure; and performing a replacement metal gate (RMG) process to transform the gate structure into a metal gate.
 9. The method of claim 8, further comprising: performing a planarizing process to remove the metal gate for forming a first metal gate on one side of the fin-shaped structure and a second metal on another side of the fin-shaped structure.
 10. A semiconductor device, comprising: a fin-shaped structure on a substrate; a gate structure on the fin-shaped structure; and a polymer stop layer on a top surface and sidewalls of the fin-shaped structure adjacent to the gate structure, wherein a top surface of the polymer stop layer is lower than a top surface of the gate structure.
 11. The semiconductor device of claim 10, further comprising: a first spacer on sidewalls of the gate structure; a second spacer on sidewalls of the first spacer, wherein the polymer stop layer is between the first spacer and the second spacer; and a source/drain region adjacent to two sides of the second spacer.
 12. The semiconductor device of claim 11, wherein the first spacer and the polymer stop layer comprise different materials.
 13. The semiconductor device of claim 11, wherein the second spacer and the polymer stop layer comprise different materials.
 14. The semiconductor device of claim 10, wherein the polymer stop layer comprises carbon, oxygen, nitrogen, hydrogen, and fluorine. 